Fix representation of negative values in Y axis

![screen shot 2014-10-27 at 11 37 03](https://cloud.githubusercontent.com/assets/3356474/4788814/437f5d10-5dbe-11e4-92ad-82376ebb6075.png)
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![screen shot 2014-10-27 at 11 44 10](https://cloud.githubusercontent.com/assets/3356474/4788811/3f6f4dc0-5dbe-11e4-8823-105e806cbc74.png)